An Investigation of Volcanic Depressions Part IV Origin of Hole-in-the-Ground, Y a Maar in Central Oregon. (Geological, geophysical, and energy investigations) by Volker Lorenz Center for Volcanology University of Oregon Eugene, Oregon A progress report of work carried out under NASA Research Grant NGR-38-005-012 Alexander R. McBirney, Principal Investigator Table of Contents Abstract. 1 Introduction. • 3 Regional geology 8 Structure. 16 Hydrogeology. 16 Geology of the Hole-in-the-Ground crater 18 Eruption products 25 Ejected blocks 29 Grooved blocks. 33 Muzzle velocities 36 Apparent fluid velocity and density at the orifice*... kO Particle concentration in the eruption cloud at the orifice 45 Eruption pressures 46 Petrographic notes on ejecta types. 48 Frequency distribution of ejecta 53 Shape and depth of the conduit as inferred from ejected blocks. 55 Distribution of ejecta 56 Volume of ejecta above pre-eruption surface 59 Subsurface geology based on evidence from drill-holes 59 Volume of pyroclastic debris below the floor of the crater 71 Geophysical results ?4 Energy requirements 82 Causes of the eruption. 88 Duration of the eruptions . 91 Summary of the history of eruptive activity 93 Comparison with other craters 9^ Comparison of the eruptions at Hole-in-the-Ground with other eruptions. 96 Comparison of the block distribution from the eruptions at Asama, Japan, and Hole-in-the-Ground....... 99 Acknowledgements 102 Selected references 103 Abstract Hole-in-the-Qround is a volcanic explosion crater or maar located in Central Oregon on the edge of Fort Rock basin. At the time the crater was formed between 13*300 and 18,000 years ago a lake occupied most of the basin and the site of the eruption was close to the water level near the shore. The crater is now 112 to 136 m. below the original groundlevel and is surrounded by a rim that rises another 33 to 63 a. higher. The volume of the crater below the original surface is only 60 percent of the volume of the ejecta. The latter contains only 10 percent juvenile basaltic material, mainly sideromelane produced by rapid quench- ing of the lava. Most of the ejected material is fine grained, but some of the blocks of older rocks reach dimensions of 8 m. The largest blocks are concentrated in four horizons and reached distances of 3.7 km. from the center of the crater. Accretionary lapilli, impact sags, and vesicu- lated tuffs are well developed. The crater was formed in a few days or weeks by a series of explo- sions that were triggered when basaltic magma rose along a northwest- trending fissure and came into contact with abundant groundwater at a depth of 300 to 300 m. below the surface. After the initial explosion, repeated slumping and subsidence along a ring-fault led to intermittent closures of the vent, changes in the supply of groundwater, and repeated accumulations of pressure in the pipe. Four major explosive events resulted from pressures of over 300 bars in the orifice of the vent. Ejection velocities during these periods reached 200 meters per second. The corresponding pressures and velocities during intervening, less violent stages were in the range of 200 to 250 bars and about 130 meters per second. The kinetic energy released during the most violent eruptions was po approximately 9 x 10 ergs and the seismic events that must have accom- panied these explosions had a magnitude of about 5» Ejecta 10 centimeters in size were thrown to heights of 2 to 3 kilometers and the eruption cloud may have reached 5 kilometers or more. The axis of eruption was slightly inclined toward the southeast; the form of the vent seems to have had a more important influence than wind. Base surges that accompan- ied some of the explosions left deposits of vesiculated tuff. The total energy derived from the basaltic magma was of the order p, of 5.7 x 10 ergs. Most of this energy went into heating of ground water and the enclosing country rocks; only a small part, possibly a tenth, was released by expansion and vaporization of the water and mechanical processes, such as crushing, acceleration and ejection of debris. Geophysical measurements indicate a domical intrusion below the crater floor and extending upward as a ring dike around the margins of the crater. Introduction Hole-in-the-Ground, a large explosion crater, is situated at the northwestern margin of Fort Rock Basin-Lane County-Central Oregon, approxi- mately 32 km. (20 miles) south of Newberry Caldera (fig. 1). Although not described in geological literature until 1960, this crater has drawn much attention recently because of its resemblance to certain small lunar craters. Owing to their comparable dimensions, Hole-in-the- Ground offers an excellent example that can be compared with Meteor Crater in Arizona. Peterson and Groh (1961) were the first to study the crater. They concluded that it was formed by a single or brief series of violent ex- plosions caused by rising magma coming into contact with water-saturated rocks. More recently (1963) these authors have described similar structures in the vicinity of Hole-in-the-Ground and pointed out a common maar-type origin. • The most recent study undertaken by Kim (1968) centers on gravimetric and geomagnetic measurements and their evaluation. The aim of this study is to give a detailed account of the crater and its genesis and to derive a better understanding of crater-forming processes. Mapping is based on aerial photographs and a topographic map especially compiled from aerial photographs. An important obstacle to detailed geological field investigation is, as Peterson and Groh (1961) have already pointed out, a uniform 60 to 70 cm. thick layer of Mazama pumice that covers most of the area. For this reason, our interpretations are based in large part on results of shallow J§ +it» •rl <M O O I i <oM (0 o rH CO 0) I. 11 0) (0 4> (3 H -rt O 33 0> «> m •H O > rH O Figure 3« View of Hole-in-the-Ground looking east. Fort Rock in the background. Figure 4. View of Hole-in-the-Ground looking northeast. I- X §3 U. ft 8 excavations and drill holes, which together with geophysical data, provide essential information on subsurface structure and rock units. Regional Geology Picture Rock Basalt The stratigraphy of the Fort Rock Basin has been compiled and correlated from scattered outcrops and numerous well logs (Hampton, 1964). The rocks range in age from Pliocene to Recent and consist almost entirely of tuffs, lavas and volcanic sediments. A generalized stratigraphic column is shown in fig. 6. The oldest rocks known in the Fort Rock Basin were described by Hampton (1964) as lava-flows and interbedded tuffs and sediments of the Picture Rock basalt. Lava-flows of this unit are dark grey, blue-grey or dark green, 3 to 16 m. thick and consist of a glassy or microcrystalline groundmass containing olivine-crystals up to 2 mm. and plagioclase crystals up to 6 mm. The interbedded pyroclastic beds and sediments are composed of stratified basaltic tuffs, pumiceous conglomerates and sand- stones. The thickness of this unit may exceed 330 m.; Hampton (1964) tentatively assigns it an age of early Pliocene. The nearest exposures of Picture Rock basalt are 38 km. to southeast of Hole-in-the-Ground at Silver Lake. It is not known whether this unit is present at depth in the Hole-in-the-Ground area. Volcanic rocks of intermediate composition unconformably overlay Picture Rock basalt at Cougar Mountain and Horning Bend (respectively 19 km. southeast and 24 km. east of Hole-in-the-Ground). The volcanic cone at Horning Bend is described by Hampton (1964) as being composed of a fine-grained blue-grey to buff andesite, whereas Cougar Mountain is composed of a glassy to fine-grained rock, probably rhyodacite. The ^^ 6 E E -I I O E 0 O s 10 ro ro ro i^l 2 i ro I ii i o I 10 ° CM O --l-f-.rf _„. , j- I M to •• 10 « I 5 0 s O 0 o o> o 6 ^ E o '-& o *o •o~ 3at O I £o tS: •> 2 • *o 5 C c flB C2D .e. O -.-i.^-1- — r 0 0 3 CD c «o 0 •£ Q. *- 6 O.S g 3 0 o o p •*- o s s X <D > O 0 0. CD I• VO ci 10 maximum thickness of these beds exceeds 330 m. at the central part of both cones, but gradually diminishes laterally. Hampton (1964) estimates their age to be middle Pliocene. 16 km. north of Hole-in-the-Ground intermediate rocks occur at Indian Butte. According to Williams (1957) porphyritic pyroxene andesite and andesitlc basalt flows strike 165-170 southeast and dip both to the west and east. These flows are presumably pre-Pliocene in age. It is not known whether lava-flows at any of these three places reached the area of Hole-in-the-Ground nor whether any vents discharging rocks of similar composition and age occur at depth near Hole-in-the-Ground. The Fort Rock Formation Unconformably overlying the volcanic rocks of intermediate composition and the Picture Rock basalt are, in order of abundance, tuffs, diatomite, basaltic agglomerate and basaltic lava, collectively named the Fort Rock Formation. The tuffs are mainly basaltic lapilli-tuffs and contain inter- bedded diatomite. Near eruptive centers the tuffs become coarse and change finally into basaltic agglomerates. Interbedded basalt-flows are of dark grey color and about 2 to 5 m. thick. On the basis of stratigraphic position, lithology and fossil evidence, the Fort Rock Formation is assigned a middle to late Pliocene age.